Emission of nitrogen oxides (“NOx”) from lean-burn engines (described below) must be reduced in order to meet emission regulation standards. Conventional three-way conversion (“TWC”) automotive catalysts are suitable for abating NOx, carbon monoxide (“CO”) and hydrocarbon (“HC”) pollutants in the exhaust of engines operated at or near stoichiometric air/fuel conditions. The precise proportion of air to fuel which results in stoichiometric conditions varies with the relative proportions of carbon and hydrogen in the fuel. An air-to-fuel (“A/F”) ratio of 14.65:1 (weight of air to weight of fuel) is the stoichiometric ratio corresponding to the combustion of a hydrocarbon fuel, such as gasoline, with an average formula CH1.88. The symbol λ is thus used to represent the result of dividing a particular A/F ratio by the stoichiometric A/F ratio for a given fuel, so that; λ=1 is a stoichiometric mixture, λ>1 is a fuel-lean mixture and λ<1 is a fuel-rich mixture.
Engines, especially gasoline-fueled engines to be used for passenger automobiles and the like, are being designed to operate under lean conditions as a fuel economy measure. Such engines are referred to as “lean-burn engines”. That is, the ratio of air to fuel in the combustion mixtures supplied to such engines is maintained considerably above the stoichiometric ratio (e.g., at an air-to-fuel weight ratio of 18:1) so that the resulting exhaust gases are “lean”, i.e., the exhaust gases are relatively high in oxygen content.
Although lean-burn engines provide enhanced fuel economy, they have the disadvantage that conventional TWC catalysts are not effective for reducing NOx emissions from such engines because of excessive oxygen in the exhaust. Attempts to overcome this problem have included operating lean-burn engines with brief periods of fuel-rich operation (engines which operate in this fashion are sometimes referred to as “partial lean-burn engines”). The exhaust of such engines is treated with a catalyst/NOx sorbent which stores NOx during periods of lean (oxygen-rich) operation, and releases the stored NOx during the rich (fuel-rich) periods of operation. During periods of rich (or stoichiometric) operation, the catalyst component of the catalyst/NOx sorbent promotes the reduction of NOx to nitrogen by reaction of NOx (including NOx released from the NOx sorbent) with HC, CO and/or hydrogen present in the exhaust.
Diesel engines provide better fuel economy than gasoline engines and normally operate 100% of the time under lean conditions, where the reduction of NOx is difficult due to the presence of excess oxygen. In this case, the catalyst/NOx sorbent is effective for storing NOx. As in the case of the gasoline partial lean burn application, after the NOx storage mode, a transient rich condition must be utilized to release/reduce the stored NOx to nitrogen. In the case of the diesel engine, this transient reducing condition will require unique engine calibration or injection of a diesel fuel into the exhaust to create the next reducing environment.
NOx storage (sorbent) components including alkaline earth metal compounds, for example oxides, such as oxides of Mg, Ca, Sr and Ba, alkali metal oxides such as oxides of Li, Na, K, Rb and Cs, and rare earth metal compounds, for example, oxides such as oxides of Ce, La, Pr and Nd in combination with precious metal catalysts such as platinum dispersed on an alumina support have been used in the purification of exhaust gas from an internal combustion engine. It is known that in air, these materials are mostly present in the form of carbonates and hydroxides, and these compounds are suitable for storing NOx. At high temperatures, the metal carbonates and hydroxides form metal oxides. Thus, as used in this specification, NOx storage “oxides” are intended to include the corresponding carbonates and hydroxides. For NOx storage, barium oxide (baria) is usually preferred because it forms nitrates at lean engine operation and releases the nitrates relatively easily under rich conditions. However, catalysts that use baria for NOx storage exhibit a problem in practical application, particularly when the catalysts are aged by exposure to high temperatures and lean operating conditions. After such exposure, such catalysts show a marked decrease in catalytic activity for NOx reduction, particularly at low temperature (200 to 350° C.) and high temperature (450° C. to 600° C.) operating conditions. In addition, NOx absorbents that include baria suffer from the disadvantage that when exposed to temperatures above 450° C. in the presence of CO2, barium carbonate forms, which becomes more stable than barium nitrate. Furthermore, barium tends to sinter and to form composite compounds with support materials, which leads to the loss of NOx storage capacity.
NOx storage materials comprising barium fixed to ceria particles have been reported, and these NOx materials have exhibited improved thermal aging properties compared to the catalyst materials described above. Despite these improvements, there is an ongoing need to improve the performance of NOx storage materials, particularly the ability of these materials to operate over a wide temperature range and to operate effectively after exposure to high temperature. It is also desirable to improve the kinetics of NOx oxidation (required in advance of NOx storage) and the kinetics of NOx reduction (required following NOx release). Thus, there is a need to provide improved NOx storage materials and methods for their manufacture.